A variety of compositions and processes have been used or suggested for use in order to impart improved corrosion resistance to chromium plated substrates to prevent the formation of rust spots when exposed to a corrosive environment. The use of nickel/chromium electrodeposits on a metal or plastic substrate to provide a decorative and corrosion resistant finish is also well known.
Traditionally, the nickel underlayer is deposited electrolytically from an electrolyte based on nickel sulfate or nickel chloride, and boric acid. This electrolyte also typically contains organic additives to make the deposit brighter and harder and also to confer leveling (i.e., scratch hiding) properties. The organic additives also control the electrochemical activity of the deposit and often duplex nickel deposits are applied where the layer closest to the substrate is more noble than the bright nickel deposited on top of it. This improves the overall corrosion performance as it delays the time required for penetration to the substrate by the corrosive environment. Typically, the total thickness of the nickel electrodeposited layer is between about 5 and about 30 micrometers in thickness.
Following the application of the nickel underlayer, a thin deposit of chromium (typically about 300 nm in thickness) is applied from a solution of chromic acid containing various catalytic anions such as sulfate, fluoride, and methane disulfonate. The chromium metal deposited by this method is very hard and wear resistant and is electrochemically very passive due to the formation of an oxide layer on the surface. Because the chromium deposit is very thin, it tends to have discontinuities through which the underlying nickel is exposed. This leads to the formation of an electrochemical cell in which the chromium deposit is the cathode and the underlying nickel layer is the anode and thus corrodes. In order to ensure even corrosion of the underlying nickel, a deposit of microporous or microcracked nickel is often applied prior to chromium plating. Thus, in the presence of a corrosive environment, the nickel will corrode preferentially to the chromium. One such process is described, for example in U.S. Pat. No. 4,617,095 to Tomaszewski et al., the subject matter of which is herein incorporated by reference in its entirety.
The half-equations of the corrosion reaction can be summarized as follows:
At the anode:Ni→Ni2++2e−At the cathode:2H2O+2 e−→H2+2 OH−
The net result is that the pores through which the corrosion occurs tend to accumulate deposits of nickel hydroxide, which detract from the appearance of the deposit. It can also be seen from the cathodic reaction that hydrogen is liberated. Electrodeposited chromium as produced from a chromic acid electrolyte is a very poor substrate for hydrogen liberation and thus the cathodic reaction is kinetically inhibited and is very slow. This means that the corrosion reaction is also very slow, which leads to an excellent corrosion performance.
A further advantage of using chromic acid based electrolytes is that exposed substrate metal which is not covered by chromium in the plating process (such as steel on the inside of tubes and exposed steel through pores in the nickel deposit or even exposed nickel pores under the discontinuous chromium layer) is passivated by the strongly oxidizing nature of the chromic acid. This further reduces the rate of corrosion.
However, chromic acid is extremely corrosive and toxic. It is also a carcinogen, a mutagen and is classified as reprotoxic. Because of this, the use of chromic acid is becoming more and more problematic. Tightening legislation is making it very difficult to justify the use of chromic acid in a commercial environment.
Chromium plating processes based on the use of trivalent chromium salts have been available since the mid-1970s and these processes have been refined over the years so that they are reliable and produce decorative chromium deposits. However, these chromium deposits do not behave the same in terms of their electrochemical properties as those deposited from a chromic acid solution.
The chromium deposited from a trivalent electrolyte is less pure than that deposited from a chromic acid solution and so is effectively an alloy of chromium. Depending on the electrolyte from which the chromium is produced, co-deposited materials may include carbon, nitrogen, iron and sulfur. These co-deposited materials have the effect of depolarizing the cathode reaction, thus increasing the rate of the electrochemical corrosion reaction and reducing the corrosion resistance of the coating. In addition, because the trivalent chromium electrolytes are not as strongly oxidizing in nature as hexavalent chromium solutions, they do not passivate any exposed substrate material, having a further deleterious effect on the corrosion performance. Thus, there remains a need in the art for a method of passivating exposed substrates that is also able to decrease the rate of the cathodic reaction during galvanic corrosion of the nickel chromium deposit.
Several attempts have been made to try to solve this problem. For example, U.S. Pat. Pub. No. 2011/0117380 to Sugawara et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of an acid solution containing dichromate ions used cathodically to deposit a passive layer onto chromium deposits from a trivalent electrolyte. However, this process does not avoid the use of toxic hexavalent chromium and actually introduces a small amount of hexavalent chromium onto the surface of the treated components.